1. Field of the Invention
The present invention relates to an exposure correcting device for use in an original reading apparatus such as a facsimile.
2. Description of the Prior Art
In an original reading apparatus in which a slit area of an original illuminated by a light source is projected in a reduced scale through a lens onto a solid-state imaging device for original reading, the amount of light on the focal plane is larger in the central area than in the peripheral area due to the cosine fourth power rule of the lens even if the original is evenly illuminated. Such uneven exposure has been corrected by increasing the intensity of illuminating light source in the peripheral area by 1/cos.sup.4 times with respect to that in the central area.
Such correction however requires a special light source, such as a halogen lamp having discrete filaments which are made longer in the peripheral area than in the central area.
On the other hand, U.S. Pat. No. 3,469,916 discloses the use of a light-shield plate, in the light path from the original to the lens, of a shape capable of compensating the light loss resulting from the cosine fourth power rule.
Such a light-shield plate is however generally difficult to manufacture of the special shape thereof and poses difficult adjustment since the distribution of exposure is changed if the mounting position is rotated from the normal position.